This research concerns events happening in the thirteenth and early fourteenth centuries in Southeast Asia, when the Mongols actively and violently interfered in southern China and Southeast Asia. The main focus will be on bilateral relations between Dai Viet and Champa, the subjects of four consecutive attempted Mongol conquests, from 1258 to 1288 C. E. Under the rule of the Mongol Great Khan M6ngke and the Yuan Emperor Kublai Khan, the Mongols invaded Dai Viet in 1258,1285 and 1287-1288 C. E., with Champa being the main target in between, in 1283-1285 C. E. This chain of Mongol invasions exposed a previously unknown point of entry into Dai Viet territory, to the south of the Vietnamese border with Champa, whereas, traditionally, the threat to Dai Viet from China has been confined to its northern border. Dai Viet's subsequent efforts, both diplomatic and military, to secure their southern frontier broke up Viet- Cham relations, and put Champa under Dai Viet's control for nineteen years, from 1307 to 1326 C. E. The loss of this piece of Cham land, the former O and Lý territories, to Dai Viet has been recorded in the Vietnamese official annals as a gift from Champa to Dai Viet, following a marriage between a Cham king and a Vietnamese princess in 1306 C. E. Alternatively, it has been attributed to Vietnam's 'Nam Tien' policy, a term describing a Vietnamese slow and continuing movement southward to occupy more land. Without relevant Cham evidences to the contrary, attention has been diverted to either the 'Nam Tien', or the 'wedding gift' theory, to interpret this loss of Cham land and the ensuing territorial conflict between Dai Viet and Champa. In doing so, other possible causes for the breakdown in Viet-Cham relations have been ignored. This thesis will seek to establish that the territorial dispute and the resulting conflict that led to the eventual demise of Champa, were direct results of the Mongols' attempted occupation of both D4ii Vi4t and Champa in the late thirteenth century.